Spice, welcome, you former lurker. First, the easy one: there's no benefit whatever to bi-wiring(aka buy-wiring)and it's electrically essentially identical to single wiring. The dividing point is simply moved back from just inside the speaker terminals to just outside the receiver terminals, which makes no significant difference.
Note that the surround speakers in a 5.1 setup are side, not back surrounds, and should be positioned only a little(maybe 2')farther back than directly to the side.
The sub question may not be as clearcut. There are a few who've made a 350-500 change and are convinced that it was money well-spent, but now it's your money.
